1. Field of the Invention
This invention relates generally to bimetal thermostats for use in electric circuits. More specifically, the invention relates to a sealed case thermostat, where, after the thermostat snaps to the open position, the thermostat remains in the open position until the power supplied to the device is turned off or removed.
2. Discussion of the Related Art
Thermostats use a bimetal blade that is essentially two strips of metal having different coefficients of expansion laminated to each other. With an increase in ambient temperature, the blade will bend away from the side containing the material with the higher coefficient of expansion. In the case of a so-called snap action blade, the blade is molded with a formed midsection so that the bending in response to increased temperature will not be gradual, but will occur suddenly once a threshold temperature is achieved. Thus, the blade will "snap" to its bent position.
These bimetal blades are then incorporated into a case and/or placed adjacent another contact so that an electrical connection will be either formed or broken as the bimetal blade bends. This type of thermostat has many applications, but has recently been used extensively in the electronics industry, mainly because of the ability to make these thermostats relatively compact.
Recently, new standards are being considered for motors, transformers, home appliances, etc., where if the device overheats, the thermostat that is mounted within the device, upon reaching the overheated condition, is now required to snap to the open or off position and is, thereafter, required to stay in the open position until either the power supply is removed from the device or a switch is used to turn off the power supply to that device.
U.S. Pat. No. 4,703,298 discloses a thermostat that includes ceramic mounting pins 4, 5 that are made of resistive material. The thermostat is an open-type device and is, therefore, subject to all kinds of atmospheric conditions. Two metal contact carriers 2, 3 are fastened onto parallel pins 4, 5. Carriers 2 and 3 are selectively moveable relative to each other on pins 4 and 5 so that the thermostat can be adjusted for a specific circuit opening temperature. Thus, the thermostat is required to be open to permit the carriers 2, 3 to move with respect to the pins 4, 5.
In use, when the bimetal 7 moves to the open position, a significant current occurs briefly in the pins to rapidly heat the ceramic PTC pin material to its high resistivity temperature to effectively completely open the circuit between carriers 2, 3. After bimetal 7 moves to the open position, a small current continues to flow in the high resistivity pins to generate heat at the stabilizing temperature of the pins. This small current flow is sufficient to maintain the bimetal 7 above its reset temperature even though the ambient temperature being monitored by the thermostat may return to its original or normal level. The thermostat thus remains open until it is manually reset, by disconnecting it from the circuit, to permit pins 3, 4 to cool to their original room temperature resistivity.
For many uses it is desirable for the thermostat to be sealed from the local ambient conditions to ensure the efficient operation of the thermostat. Sealed thermostats can be calibrated or adjusted for a specific circuit opening temperature by distorting the case at a predetermined location as is well known in the art (see, for example, U.S. Pat. Nos. 3,443,259 and 3,223,808 the disclosures of which are hereby incorporated by reference). Thus, it is an object of the present invention to provide a sealed case hold open thermostat.